U.S. Patent Nos. 4,640,352 and 4,886,118 and "Under Ground Shale Oil Pyrolysis According to the Ljunstroem Method", Chief Engineer Goesta Salomonsson, IVA, vol. 24 (1953), no. 3, pp 118-123 disclose conductive heating of subterranean formations that contain hydrocarbons to recover hydrocarbons therefrom. Conductive heating is particularly applicable to low permeability formations such as diatomites, porcelanite, coal, oil shales and other source rocks. Formations of low permeability are not amenable to oil recovery methods that require injection of fluids into the formation such as steam, carbon dioxide, or fire flooding because flooding materials tend to penetrate formations having low permeability preferentially through fractures. The injected fluids bypass most of the formation hydrocarbons. In contrast, conductive heating does not require fluid transport into the formation. Formation hydrocarbons are therefore not bypassed as in a flooding and in-situ combustion process. When the temperature of a formation is increased by conductive heating, vertical temperature profiles will tend to be relatively uniform because formations generally have relatively uniform thermal conductivities and specific heats. Production of oil in a thermal conduction process is by pressure drive, vaporization and thermal expansion of oil and water trapped within the pores of the formation rock. Oil migrates through small fractures created by the expansion and vaporization of the oil and water.
Patent '352 discloses 600.degree. C. to 900.degree. C. as a preferred temperature range for heat injection for the thermal conduction process. Electrical resistance is disclosed as a preferred heat source for the thermal conduction process. The process described in the Salomonsson article uses electrical resistance heat injectors and a heat injection rate of about 240 watts per foot. This rate of heat input would result in an injection temperature within the temperature range of about 600.degree. C. to 900.degree. C.
This heat conduction process has been known for a relatively long time, yet can not be practiced economically. Commercial applications are not economical mainly due to the long time period required to produce hydrocarbons with a reasonable number of wells. A sufficient amount of capital can not be justified by oil production that will not be realized for such a long time period.
The high cost of electrical energy is also an impediment to commercial projects using these prior art methods. Conversion of hydrocarbons to electrical energy is typically accomplished at only about 35 percent efficiency and requires a considerable capital investment. Directly burning hydrocarbons considerably lowers energy costs.
Gas fired heaters which are intended to be useful for injection of heat into subterranean formations are disclosed in U.S. Pat. No. 2,902,270 and Swedish Patent No. 123,137. These burners utilize flames to combust fuel gas. The existence of flames results in hot spots within the burner and in the formation surrounding the burner. A flame typically provides a 1650.degree. C. radiant heat source. The heaters are therefore more expensive than a comparable heater without flames. The heater of Swedish Patent 123,137 would appear to result in flameless combustion such as the present invention if the combustion air and the fuel gas were heated to a temperature above the autoignition temperature of the mixture. But due to the shallow depths of the heat injection wells disclosed in that patent, the components do not appear to be heated sufficiently to result in flameless combustion. At burner temperatures above about 900.degree. C. about 100 feet of wellbore would be sufficient to preheat the combustion air and the fuel gas for flameless combustion. Further, radiant heat transfer from the flames appears to be critical in obtaining the temperature profile indicated in FIG. 2 of the Swedish patent because little heat would be transferred from the wellbore to the formation above the portion of the borehole containing flames. Due to the existence of flames, the service life and the operating temperatures of these burners are limited.
FIG. 2 of the Swedish patent shows a temperature profile within the heat injector wellbore, but the nature of radiant heat transfer would not result in a uniform temperature profile such as this. The temperature of the casing would be significantly greater at points close to the flames. The average temperature of the heat injector would therefore realistically be considerably lower than the metallurgical limits of the well materials.
U.S. Pat. Nos. 3,113,623 and 3,181,613 disclose gas fired heat injection burners for heating subterranean formations. These burners utilize porous materials to hold a flame and thereby spreading the flame out over an extended length. Radiant heat transfer from a flame to the casing is avoided by providing the porous medium to hold the flame. But for combustion to take place in the porous medium, the fuel gas and the combustion air must be premixed. If the premixed fuel gas and combustion air were at a temperature above the autoignition temperature of the mixture, they would react upon being premixed instead of within the porous medium. The formations utilized as examples of these inventions are only up to fifty feet thick and below only about fifteen feet of overburden. The fuel gas and the combustion air are therefore relatively cool when they reach the burner. The burner would not function as it was intended if the formation being heated were significantly thicker or buried under significantly more overburden.
Natural gas fired well heaters that are useful for heating formations to temperatures sufficient for ignition of in-situ fire floods are disclosed in U.S. Pat. Nos. 2,923,535; 3,095,031; 3,880,235; 4,079,784; and 4,137,968. Provisions for the return of combustion gases to the surface are not required for ignition of fire floods. The combustion gases are intended to be injected into the formation. A long service life is also not required due to the short time period during which the burner is needed. The fuel gas and combustion air also remain relatively cool as they go down a borehole toward the burner. These burners are therefore not suitable for use as heat injectors, and do not overcome the shortcomings of the prior art heat injector burners.
It is therefore an object of the present invention to provide a method to recover hydrocarbons from a hydrocarbon containing formation using a conductive heat transfer. It is another object to provide such a process wherein more than about 75 percent of the original oil in place may be recovered. In a preferred embodiment it is an object to provide such a process which is capable of recovering hydrocarbon from a formation having a low permeability such as oil shale or diatomite.